Charge recombination via long-distance electron transfer through frozen and molten n-alkyl chains in pulse-irradiated mesomorphic phthalocyanines

Charge recombination following pulsed ionization of columnarly stacked, peripherally octa-n-alkoxy substituted phthalocyanines ("PcOCn" with n the alkyl chain length) has been studied within the time frame 10(-9) to 10(-2) s, from -100 to +200 degrees C, and for n = 5-18, corresponding to intercolumnar distances from 20 to 40 Angstrom, as determined by SAXS. The first half-life of recombination obeys an exponential dependence on the edge-to-edge distance, R, between the nearest neighbor macrocyclic cores, t(1/2) = tau(R) exp(beta(R)R), indicating intercolumnar electron tunneling to be the rate-determining step. In the solid, constant values of tau(R) = 1.5 x 10(-13) s and beta(R) = 1.1 Angstrom(-1) are reached at low temperatures for n = 5-18. t(1/2) increases at elevated temperatures, most markedly for the shortest chains. This is attributed to "premelting" beginning at the outermost carbon atoms and resulting in conformational disorder in the interfacial region between columns. In the mesophase, for which the alkyl chains are completely molten, tau(R) approximate to 6 x 10(-13) exp[0.21(eV)/k(B)T] s and beta(R) = 0.64 Angstrom(-1) for n = 5-12. In the mesophase of the PcOC18 compound recombination occurs preferentially by intercolumnar molecular ion diffusion rather than by electron tunneling. For all compounds the decay kinetics are disperse and can in many cases be described quite well by the Kohlrausch expression sigma(t) = sigma(0) exp(-t/Gamma)(alpha).